Method for making a graphic representation on objects made of a polyolefin polymer and polypropylene membrane for implementing said method

Abstract: The method for making a graphic representation on objects made of a polyolefin polymer, consists of printing a graphic representation on a face of a first film made of a stretched polyolefin polymer, associating the first film with a second film having a lower melting point than that of the first film, in order to obtain a membrane formed by the associated first and second films containing the graphic representation between them, transferring the membrane inside an injection mold and injecting a polyolefin polymer in the mold for forming the object, obtaining at least a partial melting of the second film so that the membrane forms one body with said object.

Description: The present invention relates to a method for making a graphic representation on objects made of a polyolefin polymer and to the polypropylene membrane for implementing said method. As is known, currently, for reproducing a graphic representation on a plastic object, for example on an object made of a polyolefin polymer, such as polyethylene or polypropylene, a film is printed with a known printing system (rotogravure or flexographic printing), the film being of, for example, a polyolefin polymer which after the dinking operation is manually introduced into a mold. EP 0 387 883 discloses a method for inserting labels into a mold, particularly a mold for blow-molding. The label comprises an outer transparent non-oriented plastic layer, a printed layer, printed on the rear side of said outer layer, an adhesive layer and a bonding layer suitable to be welded by the heat of the article being molded in contact with the label. This document also shows the features of the preambles of claims 1, 13 and 15. Japanese application 256 111/1990 discloses the printing of a graphic representation on a base sheet, which is covered by an outer transparent sheet. Subsequently, in a per se known manner, plastics is injected into the mold (polyethylene or polypropylene) for producing the desired object. Drawings, signs and similar, are currently printed by this method on small objects of common use. This method has a considerable disadvantage: the film perfectly integrates in the object on which it is applied only in the regions which are not printed, while on the printed film surface, the film only rests on the object surface because of the inks that constitute a "barrier" to the molecular penetration between the film and the material constituting the object. The above is the direct consequence of the fact that both the polyethylene and the polypropylene, in order to be printed, require an adapted flame surface treatment or high frequency electric discharge (crown effect) surface treatment, before printing with ink. For improving the adhesion of the inks it is also possible to use a hot chemical etching with a highly oxidant solution, such as chromic acid, oleum, and chlorosulphonic acid. As said, the above creates a "barrier" so that the printed portion of both the polyethylene and the polypropylene adhere to the material on which the representation is printed. As readily understood, this inconvenience causes the graphic representation to detach especially when the printed portion of the film is subjected to an abrasive action or denting. Furthermore, the described method only allows to print small graphic designs on small objects. In fact, the greater is the size of the design, the greater are the chances of its detachment from the object. It is not to be overlooked the fact that the bigger is the design, the more difficult is to arrange it inside the mold, especially if this operation is performed mechanically. In order to obviate to the above inconveniences, another type of transfer is used for transferring a design on a plastic object. To this purpose, by means of a calender for thermoprinting, the colors are sublimated from the paper on the surface of the object to be printed. This system allows to print a design only on small objects and on substantially plane surfaces. It is also so expensive that its use is limited to selected and limited cases. In this situation the technical aim of the present invention is to obviate to the above inconveniences. Within this technical aim, an important object of the invention is to devise a method for making a graphic representation on objects made of a polyolefin polymer, wherein the graphic representation (printed and unprinted portions) is totally integrated with the material of the object on which the design has to be printed by an effective molecular integration. A further important object of the invention is to devise a method, for making a graphic representation on objects made of a polyolefin polymer, which are extremely economical and which ensure a long durability of the representation. A further object of the invention is to provide a method for making a graphic representation on objects made of a polyolefin polymer, which can be used, without any problem, also for large graphic representations adapted to be transferred on, for example, toys, outdoor seats and tables, etc. A further object of the invention is to provide a method for making a graphic representation on objects made of a polyolefin polymer, which can be used in existing productions lines without their substantial modifications. Not least object of the invention is to provide a method for making a graphic representation on objects made of a polyolefin polymer, which allow an easy and durable application on an object, for example made of polypropylene, in order to endure thermic, mechanical and physical actions without spoiling even after a long time. The above technical aim and objects are achieved by a method for making a graphic representation according to claim 1 and a polypropylene membrane according to claim 13 Conveniently, the polypropylene membrane for making a graphic representation on objects made of a polyolefin polymer, is characterized in that it comprises a first bioriented polypropylene film, said graphic representation being printed on one face of said first film, said first film being associated, through an adhesive, with a second unstretched polypropylene film, having a melting point lower than the melting point of said first film, at least said first film having adapted additives for improving its resistance to ultraviolet rays. The present invention also relates to an injection molded object according to claim 15 comprising an external layer formed by a polyolefin film, a first lower layer formed by at least one ink adapted to define a graphic representation, a second lower layer formed an adhesive polymer, a third lower layer formed by a polyolefin polymer having a melting point lower than that of said external layer, said third layer adhering by melting to the body of said printed object. Further characteristics and advantages of the invention will be more apparent by the description of a preferred but not exclusive embodiment of a method for making a graphic representation on objects made of a polyolefin polymer, wherein the method, according to the invention, is performed by the following succession of steps. First, the film, having selected size and height, is extruded from the starting material generally constituted by granulated polypropylene. Then, the film is stretched in two directions, of which one is the (longitudinal) extruding direction and the other one is the (transversal) direction at a right angle thereof. This film is flame treated in order to ensure the adhesion of ink and/or adhesives. Then the graphic representation, which will eventually be transferred to the object to be decorated, is printed on the first film. This printing is performed in a per se known manner by rotogravure or flexographic printing. It is pointed out that, before extrusion, the starting material, which can be granulated or powder polypropylene, is stabilized with specific additives adapted to protect the polymer from the action of atmospheric oxygen and from solar light. Thanks to these additives, the film resistance to thermal aging and to the light improves from about 1-2 months to about 2-3 years. By means of a polyurethane adhesive, which is also protected from ultraviolet rays for preventing it from yellowing, the first bioriented film is associated with a second unstretched polypropylene film, having a lower melting point than that of the first film. In particular, the polyurethane adhesive is protected from UV rays thanks to the insertion of specific additives belonging to the monomeric HALS class and preferably to the monomeric methyl-hindered HALS class. In this manner, the associated first and second films form a membrane containing the graphic representation between the films, the graphic representation being thus perfectly protected both above and below. The application of the second polypropylene film to the first film provides two advantages. On one hand, makes the printing ink adhere to and cover the object completely eliminating the inconvenience of the prior art in that region where the polypropylene film did not adhere to the object. The second advantage is that, since the second film has a lower melting point than that of the first film, during injection of the material (for example polypropylene) for forming the object, the second film tends to at least partially melt thereby providing a perfect integration of the second film molecules with the polypropylene. Namely, the melting point of the second film is comprised between 120°C and 170°C, while the melting point of the first film is comprised between 120°C and 190°C. Preferably, the melting point of the second film is comprised between 145°C and 150°C, while the melting point of the first film is comprised between 160°C and 165°C. In particular, after the membrane has been obtained in the above described manner, the membrane is dinked in the desired size. The desired graphic representation is then grabbed and kept perfectly stretched and, before being applied to the mold, it is electrostatically charged. The electrostatic charge makes the membrane adhere inside the mold perfectly, so that it assumes the same shape, even if the mold does not have a perfectly plane surface, but rather has lowered and raised regions according to the object to be molded. Of course, the membrane has the first film against the mold while the second film is inside the mold so that it is affected by the injected polypropylene, not directly on the membrane, but indirectly in order not to spoil or move the membrane from its position. Example 1 The first film is constituted by a high rigidity bioriented homopolymer polypropylene having a melting point of 163°C and conveniently stabilized with a monomeric and polymeric HALS mixture. The first film is superficially flame treated for rendering it printable and is printed by a flexographic process and with inks having high resistance to the light. The first film is spread with a polyurethane adhesive added with a a monomeric methyl-hindered HALS mixture for improving the resistance to light. The first film, printed and glued, is then glued to the second film constituted by random copolymer polypropylene having a melting point of 148°C. The membrane, thus formed, after a time sufficient for the adhesive reticulation, is dinked for obtaining the exact shapes to be inserted into the mold. Example 2 Example 1 i repeated with the addition of a further protection against the aging process of the polyurethane adhesive (yellowing) due to direct exposure to UV rays. The further protection is obtained during the printing step of the graphic representation, by covering all the remaining regions of the first film which are not affected by the graphic representation, with an ink having the same color of the object on which the membrane is applied. In this manner, the polyurethane adhesive, between the two films, is not directly exposed to the solar light. The present invention also relates an apparatus for the application of the above described membrane. Namely, the apparatus comprises grip means, for example holes or suckers for sucking air, for grabbing the perfectly stretched membrane and moving it to the mold. The same support plate of the grip means, cinematically operated by numeric control, according to the known art, advantageously comprises means for electrostatically charging the membrane so that the membrane may perfectly adhere to the mold where it is arranged. Also the means for electrostatically charging may be any as far as they charge the membrane sufficiently for the above described requirements. The present invention also relates to the polypropylene membrane for implementing the above described method. The polypropylene membrane comprises a first bioriented polypropylene film. By known techniques, a graphic representation, to be associated with any injection molded object in polypropylene (for example outdoor tables, seats, toys, and similar) is printed on one face of the first film. The first bioriented polypropylene film, is associated with a second unstretched polypropylene film, by an adhesive which can be a polyurethane adhesive. Advantageously, the second film has a lower melting point than the melting point of the first film, so that it can partially melt when the polypropylene, injected inside the mold, makes contact with the second film. Advantageously, both the first film and the adhesive are stabilized with adapted additives for improving their resistance to the thermal aging and to the action of the light, particularly of the ultraviolet rays. The second film has a melting point which is substantially lower than the temperature of the polyolefin polymer, adapted to form the object on which the graphic representation has to be applied, when the polyolefin polymer makes contact with the second film, during the injection into the mold. The invention achieves the intended aims and has important and considerable advantages. It has in fact being devised a new method for making a graphic representation on objects made of a polyolefin polymer; the method achieves a perfect integration of the graphic representation support membrane with the object surface. Furthermore, the membrane applied in this manner is considerably durable and is resistant to possible scratches and abrasions. The invention thus conceived may have numerous modifications and variations, all within the inventive concept. For example, advantageously, for obtaining a further protection against the aging process of the polyurethane adhesive (yellowing) due to direct exposure to UV rays, during the printing step of the graphic representation, all the remaining regions of the first film which are not affected by the graphic representation, are covered with an ink having the same color of the object on which the membrane is applied. In this manner, the polyurethane adhesive, between the two films, is not directly exposed to the solar light. Furthermore, all the details may be substituted with technically equivalent elements. The dimensions, may be any according to the specific needs and the state of the art.

Claims: Method for making a graphic representation on objects made of a polyolefin polymer, which consists of: printing a graphic representation on a face of a first film made of a polyolefin polymer, associating said first film through an adhesive with a second film having a melting point lower than that of said first film, in order to obtain a membrane formed by said associated first and second films containing said graphic representation between them, transferring said membrane inside an injection mold and injecting a polyolefin polymer in said mold for forming said object, obtaining at least a partial melting of said second film in order that said membrane forms one body with said object, characterised in that the melting point of said second film is comprised between 120°C and 170°C and in that said first film is stretched. Method, according to claim 1, characterized in that said first film is bioriented polypropylene and that said second film is in substantially unstretched polypropylene. Method, according to at least one of the preceding claims, characterized in that at least said first film is first stabilized with additives for improving its resistance to the atmospheric oxygen and to solar light. Method, according to at least one of the preceding claims, characterized in that said second film has a melting point substantially lower than the temperature of said polymer injected into said mold when said polymer makes contact with said second film. Method, according to at least one of the preceding claims, characterized in that said adhesive is protected from the ultraviolet rays. Method, according to at least one of the preceding claims, characterized in that said membrane is electrostatically charged before being applied to said mold. Method, according to at least one of the preceding claims, characterized in that said second film is produced with an unstretched polyolefin polymer, or with a polyolefin polymer less stretched than said first film. Method, according to at least one of the preceding claims, characterized in that said first film and said second film are associated by an adhesive. Method, according to at least one of the preceding claims, characterized in that said adhesive has a polyurethane base preferably added with a a monomeric HALS mixture and more preferably added with a methyl-hindered monomeric HALS mixture. Method, according to at least one of the preceding claims, characterized in that the melting point of said first film is comprised between 120°C and 190°C, and preferably between 140°C and 175°C and more preferably between 160°C and 165°C. Method, according to at least one of the preceding claims, characterized in that the melting point of said second film is comprised between 135°C and 170°C and preferably between 145°C and 150°C. Method, according to at least one of the preceding claims, characterized in that, during the printing step of said graphic representation on said first film, the regions of said first film which are not affected by said graphic representation are covered by an ink having the same color of said object on which the membrane is applied. Polypropylene membrane for making a graphic representation on objects made of a polyolefin polymer, which comprises a first polypropylene film, said graphic representation being printed on one face of said first film, said first film being associated, through an adhesive, with a second unstretched polypropylene film, having a lower melting point than the melting point of said first film, at least said first film having adapted additives for improving its resistance to ultraviolet rays , characterized in that the melting point of said second film being comprised between 120°C and 170°C and in that said first film is stretched. Membrane, according to claim 13, characterized in that said second film has a melting point substantially lower than the temperature of said polyolefin polymer injected into a mold, when said polymer makes contact with said second film. Injection molded object comprising an external layer formed by a polyolefin film, a first lower layer formed by at least one ink adapted to define a graphic representation, a second lower layer forced by an adhesive polymer, a third lower layer formed by a polypropylene polymer having a lower melting point than that of said external layer, said third layer adhering by melting to the body of said printed object , characterised in that the melting point of said third layer being comprised between 120°C and 170°C and in that said external layer is stretched. Molded object, according to clam 15, wherein the melting point of said external layer is comprised between 120°C and 190°C, and preferably between 140°C and 175°C and more preferably between 160°C and 165°C. Molded object, according to clam 15, wherein the melting point of said third layer is comprised between 135°C and 170°C and preferably between 145°C and 150°C. Molded object, according to clam 15, wherein the melting point of the body of said molded object is lower than the melting point of said third layer.

Inventors: GRAZIOLI VITTORIO, GRAZIOLI,VITTORIO
Applicants: GRAZIOLA SPA, GRAZIOLI S.P.A.
Application year: 1994
Main classification: B29C45
IPC codes: B29C45, G09F3